@(#) README 1.5 93/11/21 21:25:34
This is the README file for the 3rd enhanced portmapper release.
Description
-----------
This README describes a replacement portmapper with access control in
the style of the tcp wrapper (log_tcp) package and a handful of other
enhancements. The portmapper provides a simple mechanism to discourage
access to the NIS (YP), NFS, and other services registered with the
portmapper.
Like all portmappers, this one is intended to be started at boot time.
Daemons that offer RPC services tell the portmapper on what port they
listen. Unlike the well-known services registered with the inetd, RPC
network port numbers may change each time the system is booted.
Whenever a client wants to use an RPC service it is supposed to first
ask the portmapper on what port the corresponding daemon is listening.
The rpcinfo command can tell you what RPC services your system offers.
As described in the features section below, the replacement portmapper
can prevent undesirable client-server interactions. In some cases,
better or equivalent alternatives are available:
The SunOS portmap that is provided with patch id 100482-02 should
close the same security holes. In addition, it provides NIS
daemons with their own access control lists. This is better than
just portmapper access control.
The "securelib" shared library (eecs.nwu.edu:/pub/securelib.tar)
implements access control for all kinds of (RPC) services, not
just the portmapper.
Reportedly, Irix 4.0.x already has a secured portmapper.
However, vendors still ship portmap implementations that allow anyone
to read or modify its tables and that will happily forward any request
so that it appears to come from the local system.
Features
--------
- optional: host access control. The local host is always considered
authorized. Access control requires the libwrap.a library that comes
with recent tcp wrapper (log_tcp) implementations.
- requests to change the portmap tables are accepted only when they
come from the local system.
- optional: requests to (un)register services that listen on privileged
ports (port < 1024) are accepted only when the requests themselves come
from a privileged port. This feature is optional because of older RPC
implementations.
- requests that are forwarded by the portmapper will be forwarded
through an unprivileged port.
- the portmapper refuses to forward requests to rpc daemons that do (or
should) verify the origin of each request: when the portmapper forwards
a request it appears to come from the local machine. At present, the
portmapper refuses to forward all RPC calls to itself, and most RPC
calls to the NFS mountd/nfsd daemons, and to the NIS daemons.
Restrictions
------------
Limiting access to the portmapper does not protect you from direct
attacks on the rpc daemons; the main task of portmap is to maintain a
table of available RPC services and of the network ports that they are
listening on. The securelib can be used to protect individual RPC
daemons, and the latest SunOS portmap+NIS fix already protects the NIS
daemons and implements limited forwarding.
On the other hand, even though a portmapper with access control only
makes an attack more difficult, it still provides an excellent early
warning system.
Origin and portability
----------------------
The sources in this distribution are derived from code on the second
BSD networking tape, which was derived from Sun's RPCSRC 4.0 code, and
from Sun's TIRPC (transport-independent rpc) distribution.
The code compiles fine with SunOS 4.1.x, Ultrix 4.x and ESIX System V
release 4.0, but I expect it will work with many other UNIX flavours.
Tested with SunOS 4.1.1; an earlier version was also tested with Ultrix
3.0. SysV.4 uses a different program that the portmapper, however;
rpcbind is the name, and it can do much more than the old portmapper.
Installation
------------
(1) Follow the instructions in the Makefile, then build the portmap and
auxiliary executables.
(2) Before killing the present portmap process, save the present
portmapper tables using the command:
./pmap_dump >table
If you kill the portmap process without saving its tables you will have
to reboot the machine.
Note: the information in the portmap tables is dynamic: For example, it
will be different after each reboot. On a Sun, it even changes each
time a windowing system is started that uses the selection service.
(3) Kill the running portmap process and start the new portmap
program. Then (still as root) initialize the portmap tables with:
./pmap_set <table
(4) If you get error messages of the form: "not registered: xxxx",
disable the CHECK_PORT feature in the Makefile, remove pmap_check.o and
rebuild the portmap program. Then proceed with step 3.
In order to revert to the original portmap daemon, kill off the running
one, restart the original one and reload its tables using the
"pmap_set" command as shown above.
Access control:
---------------
By default, host access control is enabled. However, the host that runs
the portmapper is always considered authorized. The host access control
tables are never consulted with requests from the local system itself;
they are always consulted with requests from other hosts.
In order to avoid deadlocks, the portmap program does not attempt to
look up the remote host name or user name, nor will it try to match NIS
netgroups. The upshot of all this is that only network number patterns
will work for portmap access control.
Sample entries for the host access-control files are:
/etc/hosts.allow:
portmap: your.sub.net.number/your.sub.net.mask
portmap: 255.255.255.255 0.0.0.0
/etc/hosts.deny
portmap: ALL: (/some/where/safe_finger -l @%h | mail root) &
The syntax of the access-control files is described in the
hosts_access.5 manual page that comes with the tcp wrapper (log_tcp)
sources. The safe_finger command comes with later wrapper releases.
The first line in the hosts.allow file permits access from all systems
within your own subnet; some rpc services rely on broadcasts and will
contact your portmapper anyway; and once an intruder has access to your
local network segment you're already in deep trouble.
The second line in the hosts.allow file may be needed if there are
any PC-NFS systems on your network segment.
For security reasons, the portmap process drops root privilegs after
initialization. The access control files should therefore be readable
for group or world.
Testing:
--------
Normally, only rejected requests will be reported via the syslog
daemon. Logging is done in a child process, in order to avoid
possible deadlock in case the logging code needs assistance from
the portmapper.
By default, the portmapper will be utterly silent. In fact, the portmap
daemon is not consulted that often. Sending a SIGINT signal to the
portmap process will enable the logging of all requests.
Another way to enable verbose logging is to start the daemon with the
"-v" option. See above, steps (2) and later, on how to stop and restart
the portmapper without having to reboot.
Warning: with some HP-UX versions, when verbose logging is on, the
system fills up with zombie processes. This can be fixed by compiling
with -DIGNORE_SIGCHLD (see instructions in the Makefile).
With verbose logging turned on, requests such as "ypcat" or "rpcinfo
-p" should show up with log file entries such as:
MMM dd hh:mm:ss hostname portmap[pid]: connect from x.x.x.x to getport(ypserv)
MMM dd hh:mm:ss hostname portmap[pid]: connect from y.y.y.y to dump()
Send SIGINT to the portmapper to turn the verbose logging off.
Acknowledgements
----------------
Casper H.S. Dik (casper@fwi.uva.nl) provided valuable information on
RPC security and tested an intermediate version of the portmapper with
SunOS 4.1.2. Lyford D. Rich (rich@ece.nps.navy.mil) was helpful with
porting the daemon to Ultrix 3.x. Lionel Cons (cons@dxcern.cern.ch)
solved the HP-UX problem.
Wietse Venema (wietse@wzv.win.tue.nl)
Mathematics and Computing Science
Eindhoven University of Technology
The Netherlands